Observation of biological specimens using scanning electron microscopes is widespread. In order to observe a specimen containing moisture such as a biological specimen, it must be pretreated, i.e., it is fixed, dehydrated, and dried. However, the pretreatment brings about changes in the state of the specimen. For example, it contracts or deforms. Therefore, it is impossible to observe the raw specimen.
U.S. Pat. No. 4,720,633 discloses a technique for observing specimens after pretreating them as little as possible. In this technique, a specimen is observed while maintaining the specimen chamber at a low vacuum.
In this kind of scanning electron microscope, the specimen chamber and the body of the microscope are differentially pumped. A specimen is observed while keeping the inside of the specimen chamber at a relatively low pressure, for example, a fraction of a Torr.
At such a low pressure, the moisture of the specimen is immediately frozen, and the specimen gradually dries. In this way, changes in the state of the specimen such as contraction and deformation cannot be completely prevented. Hence, it has been heretofore impossible to observe raw specimens containing moisture.